Conventionally, a three-way catalyst is disposed/provided in an exhaust passage of an internal combustion engine to purify an exhaust gas discharged from the engine. As is well known, the three-way catalyst has an oxygen storage function. That is, the three-way catalyst stores oxygen and reduces NOx when a gas flowing into the there-way catalyst (catalyst inflow gas) contains excessive oxygen. When the catalyst inflow gas contains excessive unburnt substance, the three-way catalyst releases the stored oxygen to purify the unburnt substance. Hereinafter, the three-way catalyst is also referred to as a “catalyst.”
A conventional air-fuel ratio control apparatus (conventional apparatus) comprises an upstream air-fuel ratio sensor and a downstream air-fuel ratio sensor, disposed upstream and downstream of the catalyst, respectively, in the exhaust passage of the engine. The conventional apparatus controls an air-fuel ratio (air-fuel ratio of the engine) of a mixture supplied to the engine in such a manner that an air-fuel ratio (detected upstream air-fuel ratio) represented by an output value of the upstream air-fuel ratio sensor coincides with a target upstream air-fuel ratio. This control is also referred to as a “main feedback control.”
Further, the conventional apparatus calculates a sub feedback amount so as to have an output value of the downstream air-fuel ratio sensor coincide with a “target value corresponding to the stoichiometric air-fuel ratio”, and controls the air-fuel ratio of the engine by substantially changing the target upstream air-fuel ratio based on the sub feedback amount (see, for example, patent literature No. 1). This air-fuel ratio control using the sub feedback amount is also referred to as a “sub feedback control.”